The present invention pertains to the testing of digital communications systems and pertains particularly to mask compliance testing using bit error ratio measurements.
In digital communications systems, integrity of the waveform of signals used in communication is commonly specified as an eye mask. This is true, for example, in the specification of Synchronous Optical Network (SONET) standard and the specification of the Ethernet protocol.
Typically, a sampling oscilloscope also called a Digital Communications Analyzer (DCA), is used to make eye mask measurements and guarantee that no sampled points lie in the forbidden regions of the eye mask. Because the sampling rate of a DCA is relatively slow (for example in the range of approximately 40 kilosamples per second (kS/s)) compared to the input data rate (for example in the range of 10 gigabits per second (Gb/s)), it is not possible to sample a large fraction of the incoming bits.
For example an Agilent 86100B DCA, available from Agilent Technologies, Inc., can be used to measure and test for eye diagram compliance in high-speed digital communication signals. This DCA can produce an eye diagram that consists of a sampling oscilloscope display of overlapping 0's and 1's of the incoming data stream. The oscilloscope display is triggered on a high speed clock synchronous with the data stream. Within the eye diagram, an eye mask is a predefined area in which samples are not allowed. In a typical measurement and test for eye diagram compliance, approximately 500,000 samples are used. This typically requires about 13 seconds to perform.
Alternatively, an Agilent Technologies 81250 ParBERT system, also available from Agilent Technologies, Inc., allows sample Bit Error Ratio (BER) sampling points to be chosen and compared to predetermined BER thresholds.
Bit Error Ratio testing (BERT) typically measures and compares a large number of bits (typically 1010), so good statistical accuracy can be obtained. In a BERT, a known digital sequence is produced by a pattern generator (PG). The digital data stream is captured by the BERT error detector (ED), typically after passing through some device under test. After synchronizing, a local pattern generated in the ED is compared with the captured digital data stream. The ED counts errors in the incoming data and displays the Bit Error Ratio (BER).
In a fast eye measurement performed using the Agilent 81250 ParBERT system, sample BER points are chosen and compared to predetermined BER thresholds. For example, the fast eye measurement measures the BER of a predefined number of points (1 to 32). The whole eye is not measured. The predefined number of points are each defined by a threshold and timing value relative to the starting point of the measurement. To perform a measurement, the user enters pass/fail criteria of the measurement and the BER threshold, finds the middle point of the eye with the sequence and then runs the BER.
The fast eye measurement performed using the Agilent 81250 ParBERT system is related to the DCA based eye mask measurements that are specified in the standards, but is not exactly the same. For example, decision point positions represent samples of the BER eye contour. It is not necessarily clear to a customer how to interpret these BER thresholds compared to their traditional DCA mask measurement and to set them appropriately. Also, the DCA mask has regions outside the central eye region that are not addressed by the fast eye measurement performed using the Agilent 81250 ParBERT system. Additionally, the BERT front-end of the Agilent 81250 ParBERT system is not calibrated for frequency response (unlike a DCA) and this can distort the measured distribution and result in errors in the measurement.
The optical input of a DCA can be calibrated with a swept frequency sinusoidal modulated optical signal. This sinusoidal modulation can be produced with a continuous wave (CW) optical source together with an optical modulator. Calibrated versions of modulated CW optical sources such as the Agilent 8703 Lightwave Component Analyzer are commercially available. Very broadband optical modulation can also be produced by heterodyne of two wavelength-offset CW sources. For example the OMS-2010 calibrated swept sine wave source, available from Lightwave Electronics, operates to 110 GHz. These optical sources have calibrated modulation amplitude to frequencies much higher than are required to calibrate most digital decision circuit front-ends.